Engine exhaust emissions are becoming increasingly important for engine manufacturers. Governments and regulatory agencies are enforcing ever more stringent emissions standards for many types of on-highway and off-highway vehicles. The amount of pollutants in an exhaust flow emitted from the vehicle's engine must be regulated depending on the type, size, and/or class of engine. Manufacturers must develop new technologies to meet these standards while providing high-performance, cost-effective equipment to consumers.
One method implemented by engine manufacturers to comply with the regulation of exhaust flow pollutants is the use of a selective catalytic reduction (“SCR”) catalyst to clean nitrogen oxides (“NOx”) from the engine exhaust flow. An SCR system works by releasing a reductant, such as ammonia (“NH3”), into the engine exhaust flow in the presence of a catalyst. The NH3 may be stored on the surface coating of the catalyst where it reacts with the NOx in the exhaust flow to create environmentally friendly products, such as nitrogen gas (“N2”) and water (“H2O”). The chemical reactions of the SCR process can be represented by:NH3(g)NH3(ads)  (1)4NH3(ads)+4NO+O2→4N2+6H2O  (2)4NH3(ads)+2NO+2NO2→4N2+6H2O  (3)8NH3(ads)+6NO2→7N2+12H2O  (4)4NH3(ads)+3O2→2N2+6H2O  (5)Reaction (1) describes the ammonia adsorption/desorption from the catalyst, Reactions (2)-(4) are “DeNOx” reactions that describe the reaction between the reductant and the NOx in the presence of the catalyst, and Reaction (5) describes the oxidation of the ammonia.
It is generally desired to maximize the amount of NOx in the exhaust flow converted to H2O and N2. To achieve this, the amount of NH3 stored on the catalyst's surface may be increased. However, NH3 may also be desorbed from the catalyst and carried by the exhaust flow downstream of the catalyst to a location where the NH3 is released into the atmosphere (i.e., slip). NH3 slip is undesirable because the unreacted NH3 is released into the atmosphere and wasted. The NH3 desorption rate is strongly dependent on the catalyst's temperature. As the temperature of the catalyst increases, the desorption rate of NH3 from the catalyst's surface increases exponentially.
Unlike industrial or stationary SCR applications where engines or turbines generally operate at steady state conditions, mobile SCR systems used for on-highway trucks and off-road machines are subject to transient engine speeds and loads. During low load and low temperature periods of transient cycles, a large amount of NH3 may be stored on the surface of the catalyst. As the engine load increases, the exhaust gas temperature increases, and the flow of hot exhaust gas quickly heats the SCR catalyst, thus causing the stored NH3 to desorb. This desorbed NH3 may slip into the exhaust flow and be expelled into the atmosphere.
Model-based control has been used as one method of overcoming NH3 slip, while still attempting to maintain a good NOx conversion. A model-based controller uses an internal model to calculate the proper amount of reductant required to effectively react with and reduce the NOx, but not cause slip. However, solving the differential equations that describe the SCR process, which includes Reactions (1)-(5), may be computationally expensive and difficult to implement in real-time.
One method of describing Reactions (1)-(5) in order to control an SCR process is shown in SAE paper 2004-01-0153, “Control-Oriented Model of an SCR Catalytic Converter System” (the '0153 paper) by C. M. Schar et al. Specifically, the '0153 paper discloses a control oriented SCR model that uses an Eiley-Rideal mechanism to describe the SCR process.4NH3(ads)+4NOx+O2→4N2+6H2O  (6)The '0153 paper thus limits the number of reactions that must be solved by lumping the nitrogen oxide (“NO”) and nitrogen dioxide (“NO2”) terms into a single NOx term. This simplified reaction is then used to design a model based control for an SCR device.
Although the '0153 paper may outline a method of simplifying the SCR calculations by combining Reactions (2)-(4) into a single fictional reaction, the results produced by the fictional reaction may be suboptimal. For example, the '0153 method may only account for a single global DeNOx reaction that is unable to differentiate between the NO and NO2 components, which may be undesirable in some cases. The assumption that the NO and NO2 may be lumped into a single term may only be realistic when there is no diesel oxidation catalyst or catalyzed soot filter upstream of the SCR. Due to differences in reaction rates and stoichiometric ratios between the three DeNOx reactions, using a single fictional reaction may create a need for extensive model calibration for every engine and aftertreatment configuration.
The present disclosure is directed at overcoming one or more of the problems set forth above.